How to create a new role in Alfresco 3.0?

I am using Alfresco 3.0 and i want to make a new role named "USER" who will have rights only to view and upload a document but cannot delete or edit the document.He also cant create space or content.What are the changes that are required to be made in the permissionDefinations.xml file?Do i need to change anything in any other file?Thanks in advance……………..